High velocity atomic particle beam exit window

ABSTRACT

A composite metal foil exit window for use in an atomic particle accelerator. The exit window is formed from cohesively joined, bonded, or chemically attached layers of a metal having a low energy loss or stopping power for atomic particles, and an unreactive material. An exit window so constructed has improved chemical stability in a hostile environment and yet allows passage of a great number of high velocity atomic particles with small energy loss. Preferred embodiments of the exit window include those having cohesively joined layers of: aluminum and aluminum oxide.

United States Patent [191 [111 3,778,655 1 51 Dec. 11, 1973 Luce 1 1 HIGH VELOCITY ATOMIC PARTICLE BEAM EXIT WINDOW [76] Inventor: George A. Luce, 1512 Villanova Dr., Austin, Tex. 78758 [22] Filed: May 5, 1971 [21] Appl. No.: 140,499

[52] US. Cl. 313/63, 313/74 [51] Int. Cl. H05h 7/00 [58] Field of Search 313/74, 63

[56] References Cited UNITED STATES PATENTS 3,239,706 3/1966 Farrell et al. 313/74 3,211,937 10/1965 Hester et a1. 313/74 2,449,872 9/1948 Brasch et al..... 313/74 3,641,454 2/1972 Krawetz 317/74 FOREIGN PATENTS OR APPLICATIONS 327,152 3/1930 Great Britain 313/74 Primary Examiner-Roy Lake Assistant Examiner-Darwin R. Hostetter Attorney-Lowell C. Bergstedt, Walter C. Ramm, Charles H. Thomas, Jr. and Helmuth A. Wegner [57] ABSTRACT A composite metal foil exit window for use in an 4 Claims, 3 Drawing Figures PATENIEIUUECH I975 v 3778655 INVENTOR.

GEORGE A. LUCE HIGH VELOCITY ATOMIC PARTICLE BEAM EXIT WINDOW This invention relates to a composite metal foil exit window for use in an atomic particle accelerator. More particularly, the exit window is formed from cohesively joined layers of a metal providing low particle energy loss or stopping power and a material with low chemical reactivity. An exit window so constructed has improved chemical and structural stability in a hostile environment and still allows the passage of a great number of high velocity atomic particles with small energy loss.

BACKGROUND OF THE INVENTION Since the introduction of atomic particle accelerators in which beams of atomic particles are generated in an evacuated structure and passed through an interface into the surrounding atmosphere, several problem areas have been recognized in the construction of a beam exit window at the interface. Most important, the exit window material must allow passage of the beam of atomic particles without'sevcre degradation of the energy of the particles. That is, the energy of the atomic particles in the beam transmitted through the exit window must be a significant fraction of the energy of the atomic particles in the beam striking the exit window. This characteristic is described by a statement of the energy loss per unit thickness of the window. There is a calculable relationship between the energy loss and the' thickness of the exitwindow and between the energy loss and the atomic number of the metal from which the exit window is constructed, as well as between the energy loss and the initial energy of the beam of particles. In constructing an exit window, therefore, it is desirable that both the atomic number of the window material and the window thickness be small. To achieve minimal window thickness, the metal used is preferably highly ductile so that it can-be rolled into an extremely thin sheet of metal foil. For these reasons, metals such as aluminum and magnesium, and particularly aluminum have been extensively used in the construction of exit windows for atomic particle accelerators. In addition to having other favorable characteristics, these metals, furthermore, are highly thermally conductive. This feature aids in the dissipation of heat generated by the energy loss of the atomic particles that are transmitted through the exit window. The conventional metals heretofor mentioned have certain inherent disadvantages, however, in that they exhibit a tendency to react with substances in the atmosphere. Furthermore, the natural tendency for chemical reaction is frequently increased by exposure to the hostile environment created by the interaction of the transmitted beam and the substances in the surrounding atmosphere. For example, in an electron accelerator the electron beam entering the atmospherecreates a hostile environment at the exterior surface of theexitwindow by promoting chemical reactions involving oxygen and nitrogen to form corrosive compounds, such as ozone and nitrogen oxides. The electrons also create other corrosive substances by causing reaction of water vapor in the atmosphere with the nitrogen and oxygen compounds. The presence and action of these corrosive compounds at the surface of the window foil causes deterioration of the foil and destruction of its vacuum sealing properties. Additionally, the corrosive compounds produced in the surrounding atmosphere,

in reaction with an aluminum foil window, create compounds on the-aluminum surface which are less thermally conductive than is the aluminum foil. With continued bombardment by the particle beam excessive heating of the window foil causes failure through the creation of mechanical stresses.

Attempts have been made to substitute other materials in place of metal foil windows having low energy loss. Titanium foils have been substituted for aluminum foils because the mechanical strength and chemical inertness of titanium is much greater than that of aluminum. Because of the superior strength of titanium, titanium foils of 0.0005 inches to 0.0006 inches in thickness may be substituted for aluminum foils of 0.001 inches to obtain equivalent energy loss. The use of titanium provides superior corrosion resistance, but the heat conductivity for titanium foil windows is poorer than that of aluminum so that mechanical failure may easily occur if the atomic particle beam is not properly handled as it strikes the foil. That is, if the atomic particle beam power density is above a certain level, window foil failure may occur due to structural weakness induced by localized heating in the titanium metal.

While previous attempts to improve exit windows in atomic particle accelerators have been directed toward constructing windows of materials having favorable characteristics and minimizing the unfavorable characteristics, a different approach to the problem is disclosed herein, In this invention two or more materials, each having favorable properties, are combined in cohesively joined layers to form a vastly improved exit window.

Accordingly, it is an object of this invention to produce an exit window for an atomic particle accelerator which provides a low energy loss in a beam of atomic particles, a characteristic which is termed low stopping power, and which simultaneously possesses the properties of high mechanical strength, good vacuum sealing properties, resistence to detrimental effects from both thermal and chemical influences, and high thermal conductivity.

Another object of this invention is to produce an atomic particleexit window which exhibits improved vacuum sealing characteristics by virtue of a reduction in the normal porosity and gaseous diffusion rate of the window material.

It is a further object to produce an exit window in an atomic particle accelerator which will maintain its usefulness for extended periods of use, rather than merely days or hours of use as has been observed in convenergy loss for charged particles and an unreactive material. The invention may also be considered as, in an atomic particle accelerator employing a beam of accelerated atomic particles generated in an evacuated structure and passed out of said evacuated structure into a chemically reactive environment, the improvement comprising a' composite foil exit window transversly positioned with respect to said beam and sealed across an opening in said evacuated structure and having interior and exterior surfaces and having a layer of an unreactive material at said exterior surface bonded to a layer of metal having a low energy loss at said interior surface.

Whether or not a metal has a low energy loss may be determined by comparing it with aluminum. A window material compares favorably with aluminum if a thickness of the former required to withstand the mechanical stresses exhibits a total energy loss for emergent particles comparable to that of an aluminum window. If the total energy in a beam of atomic particles emerging from a particular metal foil is greater than about 85 percent of the total energy in a beam of atomic particles emerging from an aluminum foil of equal thickness where the two beams generated are of identical energy and electron density, then that particular foil may be said to have a low energy loss for charged particles and a low stopping power. Furthermore, a foil may be said to be unreactive if it undergoes no significant damaging chemical reaction when bombarded by a beam of atomic particles with at least one surface exposed to the atmosphere over a prolonged period of time, such as several weeks. Damaging chemical reactions are those type of reactions which render a metal foil porous or unable to sustain a given pressure differential or which introduce areas of significantly decreased thermal conductivity.

This invention may be more fully illustrated in the accompanying drawings in which:

- FIG. 1 is a diagram of an electron accelerator utilizing an exit window improved according to this invention;

FIG. 2 is a perspective view illustrating the construction of the exit window of FIG. 1;

FIG. 3 is a plan diagram of a cyclotron utilizing the exit window of this invention.

Referring now to FIG. 1 there is shown a conventional electron accelerator having a cathode 11 which supplies electrons. The electrons are accelerated in a beam 13 through the evacuated chamber 12 by means of electrodes 30 located at the interior walls of chamber 12. Chamber 12 is evacuated by a vacuum pump (not shown) through a vacuum line 29. The electron beam 13 leaves the evacuated chamber 12 and enters the atmosphere at an interface 16. Interface 16 is comprised of a metal foil exit window 18 vacuum sealed by gasket or ring seal 33 and-held in place by a retaining ring 14 and bolts 19. The metal foil 18 is supported somewhat against the pressure differential between the atmosphereand the evacuated chamber 12 by support vanes 34 fastened to frame 35. The interior surface of exit window 18 is in contact with the vanes 34.

Upon entering the atmosphere from chamber 12, the electron beam may be utilized for a variety of purposes, such as sterilization, preservation, sewage treatment, curing coating compositions, such as paints, varnishes, etc., and other conventional uses. The exit window 18 is a composite foil exit window having a layer 20 of a metal having a low energy loss and good thermal conductivity and a layer 21 of an unreactive material. The layers 20 and 21 are cohesively joined or bonded throughout their mutual surfaces of contact. The metal forming the layer 20 is highly thermally conductive so that heat is distributed quickly and severe localized hot spots develop only rarely. Moreover, the metal forming layer 20 preferably has a low atomic number, as such metals tend to'have a low energy loss. Aluminum is an excellent material from which to form the layer 20. Other ductile, high conductivity materials, such as copper and silver may also be utilized to advantage in certain variations of this concept in exit window construction.

Various materials may be used to form the unreactive layer 21. For example, an anodized aluminum foil may be used to form the exit window 18. In this construction, aluminum metal forms the layer 20 while a layer of aluminum oxide forms the layer 21. Various other combinations of appropriate materials may be used to form the exit window 18, but for the purpose of illustration, the exit window 18 in the drawings may be assumed to be comprised of a layer 20 of aluminum about 0,001 inches thick to which is bonded a layer 21. The relative thicknesses of the layers 20 and 21 may vary considerably. For example, the aluminum layer 20 may be 0.001 inches thick and the layer 21 may be about 0.0001 inches thick. Alternatively, the layer 21 may be from about 000025 to about 0.0005 inches thick and the aluminum layer may be about 0.0001 inches thick.

The exit window described may be used in atomic particle acceleration devices other than electron accelerators. For example, an exit window 18' of the type heretofore described is utilized in the cyclotron of FIG. 3. The cyclotron 22 is comprised of two hollow generally semi-circular electrodes 23 and 24. The electrodes 23 and 24 are separated slightly and enclosed in a vacuum chamber 31 bounded by walls 32 and exit window 18'. A powerful radio frequency oscillator 25' provides an alternating electric field between the electrodes 23 and 24 at a frequency of from 2 to 20 megacycles per second. Positive ions leave the positive ion source 27 located at the center of the evacuated chamber 31 when electrode 24 is at its maximum negative potential and electrode 23 is at its maximum positive potential. Ions will at this time be accelerated toward electrode 24 and will enter this electrode. In the interior of an electrode, the positive ion is subject to no electric field. The changing electric field causes the ions to spiral outward at ever increasing speeds. A particle deflection plate 26 deflects the positive ion beam 28 toward the exit window 18', which may be constructed similar to the exit window 18 of FIG. 2.

In addition to use in the atomic particle accelerators illustrated, the exit window of this inventionmay be utilized in virtually any other atomic particle accelerator in which an atomic particle beam is passed through an exit window from an evacuated chamber to a chemically reactive fluid ehvirohment. The exit window of this invention may therefore also be used in van de Graaff generators, linear accelerators, synchrocyclotrons, betatrons, synchrotrons, and other devices.

The exit windows of this invention may be manufactured by subjecting a base layer of a metal having a low energy loss to a conventional coating treatment, such as anodization, passivation, or conversion coating, in order to obtain a layer of an unreactive material bonded to the base layer.

The thickness of each layer would be adjusted for optimum characteristics.

The choice and number of alternative materials for use in the exit window disclosed herein is rather extensive. One might construct exit windows of foil layers of aluminum and aluminum oxide, as well as many other combinations. The combination of layers of metal chosen may be designed for a specific atomic particle beam intensity, hostile environment, operating temperature condition, or beam energy requirement. Additionally, a particular combination may be chosen to provide improved vacuum sealing properties in addition to the previously cited factors. That is, a particular layer of overcoating material may reduce the porosity and gaseous diffusion rate of the base material.

A There are several different particular metal foil exit window constructions that have been successfully used in accordance with this invention. One such construction is a base layer of aluminum foil about 0.001 inches thick, overcoated with an aluminum oxide layer 0.0001 inches or less thick. This exit window may be manufactured by an anodizing process by placing the aluminum foil in a 40 grams per liter solution of CrO and connecting the aluminum foil as an anode when a steel cathode is also present in the solution. A voltage differential of 35 volts is maintained between the anode and cathode over a period of about minutes at a temperature of about 95 F. The coated aluminum foil, when rinsed and dried after removal from the solution, is ready for immediate use as an exit window in an atomic particle accelerator. If desired, the anodized coating may be sealed" by conventional means. Anodized coatings are often sealed by filling the pores with some material, for example a dye for decorative purposes.

The specific exit window construction and method of manufacture described have been given for clearness of understanding only, and no unnecessary limitations should be construed therefrom. Further, it is i'mmaterial for the general purposes of this invention, as to the size or shape of the window foils, the exact thickness of the several materials, or as to the method of supporting the foil as an exit wiridow.

Furthermore, the origin of the beam is not a material consideration to this invention, nor is the scanning or other motion of the beam or the stationary nature of the beam with respect to the exit window a material consideration.

I claim as my invention:

1. A composite foil exit window for use in an atomic particle accelerator and capable of passing at least about percent of the energy in an atomic particle beam passing therethrough and comprising cohesively joined layers of aluminum and aluminum oxide formed from a common mass wherein the layer of aluminum oxide is from about 0.0001 to about 0.00025 inches in thickness.

2. The exit window of claim 1 wherein the combined thickness of the aforesaid layers is no greater than .0011 inches.

3. ln an atomic particle accelerator employing a beam of accelerated atomic particles generated in an evacuated structure and passing out of said evacuated structure into a chemically reactive environment, the improvement comprising a composite foil exit window transversely positioned with respect to said beam and sealed across an opening in said evacuated structure and having interior and exterior surfaces and having layers formed from a common mass including a layer of aluminum oxide from about 0.0001 to about 0.00025 inches in thickness at said exterior surface cohesively joined to a layer of aluminum at said interior surface, wherein the total energy in a beam of atomic particles emerging at said exterior surface is at least about 85 percent of the total energy in a beam of atomic particles entering said exit window at said interior surface.

4. The apparatus of claim 3 further characterized in that said atomic particle accelerator is an electron accelerator. 

1. A composite foil exit window for use in an atomic particle accelerator and capable of passing at least about 85 percent of the energy in an atomic particle beam passing therethrough and comprising cohesively joined layers of aluminum and aluminum oxide formed from a common mass wherein the layer of aluminum oxide is from about 0.0001 to about 0.00025 inches in thickness.
 2. The exit window of claim 1 wherein the combined thickness of the aforesaid layers is no greater than .0011 inches.
 3. In an atomic particle accelerator employing a beam of accelerated atomic particles generated in an evacuated structure and passing out of said evacuated structure into a chemically reactive environment, the improvement comprising a composite foil exit window transversely positioned with respect to said beam and sealed across an opening in said evacuated structure and having interior and exterior surfaces and having layers formed from a common mass including a layer of aluminum oxide from about 0.0001 to about 0.00025 inches in thickness at said exterior surface cohesively joined to a layer of aluminum at said interior surface, wherein the total energy in a beam of atomic particles emerging at said exterior surface is at least about 85 percent of the total energy in a beam of atomic particles entering said exit window at said interior surface.
 4. The apparatus of claim 3 further characterized in that said atomic particle accelerator is an electron accelerator. 